1. Field of the Invention
The present invention relates to a vehicle body reinforcing member that can demonstrate excellent impact resistance during a vehicle body collision and can also reduce the weight of the vehicle body. More specifically, the present invention relates to a vehicle body reinforcing member that can further improve impact resistance during a vehicle body collision by increasing rigidity and improving buckling resistance.
2. Description of the Related Art
In the automobile industry in recent years, the demand for vehicle body safety has increased, and the development of technologies to protect occupants during a collision has been promoted. In conjunction with these trends, the structure of every part of the vehicle body, including the doors, has been revised to improve occupant protection performance, and the use of reinforcing members to protect occupants has been studied.
For example, FIG. 1 is a drawing that shows a vehicle body structure that uses reinforcing members in an automobile door as door impact bars. A door 1 is an automobile door that is structured such that a window frame 3 is integrally formed as a one-piece with an upper portion of a door inner 2, and a door outer (not shown in the figure) is joined to an outer side of the door inner 2. Further, a front edge of the door 1 (on the left side of the figure) is attached to a vehicle body 4 by upper and lower hinges 5a, 5b. At a rear edge of the door (on the right side of the figure), a door lock 6 is provided at a middle height level of the door inner 2. This structure allows the door to open and close freely, and to be held by the door lock 6 when closed.
The door impact bars (reinforcing members) 7 have brackets 8 at both ends to join them to the vehicle body. In the structure of the reinforcing members shown in FIG. 1, the brackets 8 are joined to the hinges 5 and the door lock 6. Therefore, the structure is such that one of the door impact bars 7 is mounted between the portion where the upper hinge 5a is attached and the portion where the door lock 6 is attached, and the other door impact bar 7 is mounted between the portion where the lower hinge 5b is attached and the portion where the door lock 6 is attached.
The structure of the reinforcing members shown in FIG. 1 is such that the brackets that are used for joining the reinforcing members to the vehicle body are provided at the ends of tubes. However, another structure that may be used is a so-called bracketless structure in which the brackets are not provided at both ends of the reinforcing members, and both ends of the reinforcing members are joined directly to the vehicle body.
Recently, due to automobile fuel economy regulations and the need to reduce costs, the demand to reduce the weight of an automobile body has become stronger. Therefore, the demand to promote weight reduction has become stronger even for reinforcing members that increase safety. A variety of proposals have been made to address this demand.
In Japanese Patent No. 2811226, proposed is a steel tube for reinforcing a vehicle body that reduces weight by using a high-strength steel tube or steel sheet as a reinforcing member. The steel tube for reinforcing a vehicle body has a tensile strength of at least 120 kgf/mm2 (1180 MPa) and an elongation of at least 10%, and it is used as a door impact bar that is a steel tube for reinforcing a door, or as a bumper core material, and the like. The above characteristics of the steel tube is perceived to result in that by selecting a steel tube shape in relation to the use conditions, the steel tube for reinforcing a vehicle body can be made lighter in weight and can effectively absorb impact energy.
Furthermore, in Japanese Patent Application Publication No. 2002-144872, a door structure is proposed in which an impact bar is arranged such that it is inclined in the longitudinal direction of the vehicle body and a portion of the impact bar that faces a door outer panel is curved along and in the proximity of a curved surface of the door outer. If the proposed door structure is used, while tensile stiffness and bending resistance in relation to an impact load are imparted to the impact bar, compression resistance is additionally obtained, ensuring sufficient strength without using such methods as enlarging the cross section shape of the impact bar, increasing its thickness, and the like.
Also, in Japanese Patent Application Publication No. 04 (1992)-280924, a method is disclosed for heating a straight steel pipe by direct electric resistance heating and hardening the steel pipe by spraying it with liquid coolant while pressing a die such that the steel pipe takes a desired curved shape, so as for a door guard beam to follow the contour of a rounded shape of a door by bending the pipe. The method focuses on the fact that, for reasons of automobile design, an automobile door tends to have a rounded shape.